Ferredoxins substitutes

Pyruvate ferredoxin oxidoreductase. Within Clostridia and other strict anaerobes this enzyme catalyzes reversible decarboxylation of pyruvate (Eq. 15-35). The oxidant used by clostridia is the low-potential iron-sulfur ferredoxin.320 3203 Clostridial ferredoxins contain two Fe-S clusters and are therefore two-electron oxidants. Ferredoxin substitutes for NAD+ in Eq. 15-33 but the Gibbs energy decrease is much less (-16.9 vs - 34.9 kj / mol. for oxidation by NAD+). 

Recent x-ray studies on reduced flavodoxins (109), on the other hand, reveal a dihydroflavin moiety which appears largely planar at 2.5 A resolution. But there is one main fact one should not forget in the context of flavodoxins These proteins do serve as ferredoxin substitutes and are unique in being involved exclusively with one-electron transfer, i.e. they are l/l-flavoproteins (see above). Furthermore, delocalisation of the N(5)-nonbonding electrons in flavohydroquinone can be inhibited in a largely planar flavin nucleus, if only the N(5)-H and N(10)-R bonds are kept out of the plane. Of course, apoproteins... 

The reductions are effected in nature by ferredoxin (p. 1102). This behaviour can be reproduced surprisingly well by simpler, model compounds. Some of the best known of these are obtained by the addition of axial groups to the square-planar complexes of Co with Schiff bases, or substituted glyoximes (giving cobaloximes) as illustrated in Fig. 26.7. The reduced Co species of these, along with vitamin... 

In Desulfovibrio ferredoxins a general binding motif can be outlined, taking into account the presence of only one [3Fe-4S], one or two [4Fe-4S] cores, the substitution of one cluster by a S-S bridge, or even the replacement of the second cysteinyl residue by an aspartic acid (as in D. vulgaris Miyazaki Fdl and D. africanus Fdlll), which can, in certain conditions, coordinate a fourth iron atom to build a [4Fe-4S] 88) (Fig. 6). 

Fig. 9. Cysteine residues involved in the Cys-to-Ser substitutions carried out on [2Fe-2S] proteins. The reducible site of the center was identified by NMR in the case oi Anabaena ferredoxin (42).

Fig. 10. Comparison between the g values of Cys-to-Ser mutants and wild-type [2Fe-2S] proteins, in cases where the substitution takes place at (A) the reducible site Emd (B) the nom-educible site. The following symbols indicate the various proteins , Anahaena 7120 vegetative ferredoxin , human ferredoxin , Frd B subunit in E. coli fumEirate reductase , Clostridium pasteurianum ferredoxin. Filled and empty symbols indicate the wild-t3ipe and mutant proteins, respectively.

The work described above was greatly aided by the provision of samples from colleagues D. gigas Ni(II)-substituted rubredoxin, I. Moura and J. J. G. Moura T. thermophilus ferredoxin, J. A. Fee VFe and MoFe nitrogenase proteins from A. vinelandii, B. J. Hales. Research in the author s laboratory is supported by grants from NIH (GM33806) and NSF (DMB8796212) and an Alfred P. Sloan Research Fellowship. 

X.X Acting on reduced ferredoxin as donor. 2. 7. 8.x Transferases for other substituted... 

Fe—2S] ferredoxins, 38 238-239 xylene monooxygenase, 38 239 Pseudorotation, square-planar substitution reactions, 34 239-247 5-coordinate intermediate, 34 245... 

Covalency of the fag iron orbitals with sulfur implies that the Moss-bauer spectra of these proteins will be sensitive to ligand changes between the members of the plant-type ferredoxins. That is, the substitution of tyrosine or histidine for cysteine as a ligand is certain to cause a change in isomer shift which is not observed for these proteins. 

Three modifications of the conventional oxidative citric acid cycle are needed, which substitute irreversible enzyme steps. Succinate dehydrogenase is replaced by fumarate reductase, 2-oxoglutarate dehydrogenase by ferredoxin-dependent 2-oxoglutarate oxidoreductase (2-oxoglutarate synthase), and citrate synthase by ATP-citrate lyase [3, 16] it should be noted that the carboxylases of the cycle catalyze the reductive carboxylation reactions. There are variants of the ATP-driven cleavage of citrate as well as of isocitrate formation [7]. The reductive citric acid... 

Activation of an M-CO bond for nucleophilic substitution in anion radical metallo-complexes appears to be quite a general effect (Kaim 1987 Mao et al. 1989,1992 Shut et al. 1995 Klein et al. 1996). Such activation seems to be the basis of metal-cluster catalytic activity. The iron-sulfur cluster (Bu4N)2Fe4S4(SPh)4 deserves to be mentioned here. The cluster is considered as a ferredoxin model (Inoue Nagata 1986) it catalyzes an electron transfer from //-butyl lithium or phenyl lithium to 5-phenyl thiobenzoate or phenylbenzoate (Inoue Nagata 1986). 

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